The present invention relates to a casing for an electromagnetic radiation source, in particular an X-ray source, which includes a window making it possible to eliminate extrafocal radiation.
The invention is quite particularly applicable, in the medical field, to X-ray imaging devices.
As shown in FIG. 1, which represents the part of the window of an X-ray source of the prior art, the X-ray source comprises a cathode 10 and an anode 11 which are contained in a casing 12 that is transparent to X-rays. The assembly comprising of the cathode 10, the anode 11 and the casing 12 is in turn contained in a casing 13 that is opaque to X-rays, with the exception of a part located facing the X-ray beam emitted by the anode 11, which comprises of a window 15 made of material that is transparent to X-rays. The gap between the transparent casing 12 and the opaque casing 13 is filled by oil 16 used for insulation and for cooling the X-ray source.
As is well-known, the cathode 10 emits electron radiation which strikes the rotating anode 11 which re-emits an X-ray beam from a focal surface. The X-ray beam emitted by the anode 11 comprises radiation output by this focal surface, and also extrafocal parasitic radiation. This extrafocal parasitic radiation must be eliminated, preferably as close as possible to the emission source.
Conventionally, in order to eliminate this extrafocal radiation, as shown in FIG. 1, a conical element 30 or diaphragm made of material that is opaque to X-rays, for example made of lead, and provided with a central opening 31 for passage of the X-ray beam was provided. The diameter of the opening 31 is a compromise between obtaining a wide field and eliminating extrafocal radiation. However, in some cases, it was desirable to obtain smaller fields, for example by using a smaller focal surface. However, in this case, the diameter of the opening 31 is no longer adapted to the emitted X-ray beam and, in particular, no longer eliminates the extrafocal radiation. Conventionally, use was then made, in order to eliminate the extrafocal radiation, of extrafocal fingers 32, made of material that is opaque to X-rays, for example made of lead, which were brought into position both laterally and longitudinally by a rod system 33, in accordance with the desired field. Such a system is mechanically complex, expensive and furthermore does not make it possible to collimate the X-ray beam as close as possible to the focus.